Mug warmer



FIG. 1 is a front elevation of a mug warmer showing our new design;

FIG. 2 is a back elevational view thereof;

FIG. 3 is a front perspective view thereof; and,

FIG. 4 is a back perspective view thereof.

The details shown in broken lines are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a mug warmer, as shown and described. 